This invention relates to an epoxidation process using a carbonate-containing modifier. The process comprises reacting olefin, hydrogen, and oxygen in the presence of a modifier and a catalyst comprising a noble metal and a titanium zeolite. The modifier is (a) calcium carbonate in the presence of carbon dioxide or (b) ammonium bicarbonate. Surprisingly, the process results in higher activity and selectivity to epoxide, as well as increased catalyst life.
Many different methods for the preparation of epoxides have been developed. Generally, epoxides are formed by the reaction of an olefin with an oxidizing agent in the presence of a catalyst. The production of propylene oxide from propylene and an organic hydroperoxide oxidizing agent, such as ethyl benzene hydroperoxide or tert-butyl hydroperoxide, is commercially practiced technology. This process is performed in the presence of a solubilized molybdenum catalyst, see U.S. Pat. No. 3,351,635, or a heterogeneous titania on silica catalyst, see U.S. Pat. No. 4,367,342. Hydrogen peroxide is another oxidizing agent useful for the preparation of epoxides. Olefin epoxidation using hydrogen peroxide and a titanium silicate zeolite is demonstrated in U.S. Pat. No. 4,833,260. One disadvantage of both of these processes is the need to pre-form the oxidizing agent prior to reaction with olefin.
Another commercially practiced technology is the direct epoxidation of ethylene to ethylene oxide by reaction with oxygen over a silver catalyst. Unfortunately, the silver catalyst has not proved very useful in epoxidation of higher olefins. Therefore, much current research has focused on the direct epoxidation of higher olefins with oxygen and hydrogen in the presence of a catalyst. In this process, it is believed that oxygen and hydrogen react in situ to form an oxidizing agent. Thus, development of an efficient process (and catalyst) promises less expensive technology compared to the commercial technologies that employ pre-formed oxidizing agents.
Many different catalysts have been proposed for use in the direct epoxidation of higher olefins. For example, JP 4-352771 discloses the epoxidation of propylene oxide from the reaction of propylene, oxygen, and hydrogen using a catalyst containing a Group VIII metal such as palladium on a crystalline titanosilicate. U.S. Pat. No. 5,859,265 discloses a catalyst in which a platinum metal, selected from Ru, Rh, Pd, Os, Ir and Pt, is supported on a titanium or vanadium silicalite. Additionally, it is disclosed that the catalyst may also contain additional elements, including Fe, Co, Ni, Re, Ag, or Au.
One disadvantage of the described direct epoxidation catalysts is that they are prone to ring-open the epoxide product under standard reaction conditions to form less desirable ring-opened by-products such as glycols or glycol ethers. U.S. Pat. No. 6,008,388 describes a direct olefin epoxidation process in which the selectivity for the reaction of olefin, oxygen, and hydrogen in the presence of a noble metal-modified titanium zeolite is greatly enhanced by the addition of a nitrogen compound to the reaction mixture. Also, increasing the catalyst activity as well as maximizing the catalyst life are important objectives in order to achieve an economical process. As with any chemical process, it is desirable to develop new direct epoxidation methods and catalysts.
In sum, new processes for the direct epoxidation of olefins are needed. Especially desirable are processes that increase activity, selectivity to the epoxide, as well as improving catalyst life. I have discovered an effective, convenient epoxidation process that reduces unwanted ring-opened products, increases catalyst activity, and improves catalyst life.
The invention is an olefin epoxidation process that comprises reacting olefin, oxygen, and hydrogen in the presence of a modifier and a catalyst comprising a noble metal and titanium zeolite. The modifier is either (a) calcium carbonate in the presence of carbon dioxide or (b) ammonium bicarbonate. This process surprisingly gives significantly reduced ring-opened by-products, and increased catalyst activity and life compared to processes that do not use the modifier.
The process of the invention employs a catalyst that comprises a noble metal and titanium zeolite. Suitable titanium zeolites are those crystalline materials having a porous molecular sieve structure with titanium atoms substituted in the framework. The choice of titanium zeolite employed will depend upon a number of factors, including the size and shape of the olefin to be epoxidized. For example, it is preferred to use a relatively small pore titanium zeolite such as a titanium silicalite if the olefin is a lower aliphatic olefin such as ethylene, propylene, or 1-butene. Where the olefin is propylene, the use of a TS-1 titanium silicalite is especially advantageous. For a bulky olefin such as cyclohexene, a larger pore titanium zeolite such as a titanium zeolite having a structure isomorphous with zeolite beta may be preferred.
Titanium zeolites comprise the class of zeolitic substances wherein titanium atoms are substituted for a portion of the silicon atoms in the lattice framework of a molecular sieve. Such substances are well known in the art.
Particularly preferred titanium zeolites include the class of molecular sieves commonly referred to as titanium silicalites, particularly xe2x80x9cTS-1xe2x80x9d (having an MFI topology analogous to that of the ZSM-5 aluminosilicate zeolites), xe2x80x9cTS-2xe2x80x9d (having an MEL topology analogous to that of the ZSM-11 aluminosilicate zeolites), and xe2x80x9cTS-3xe2x80x9d (as described in Belgian Pat. No. 1,001,038). Titanium-containing molecular sieves having framework structures isomorphous to zeolite beta, mordenite, ZSM48, ZSM-12, and MCM-41 are also suitable for use. The titanium zeolites preferably contain no elements other than titanium, silicon, and oxygen in the lattice framework, although minor amounts of boron, iron, aluminum, sodium, potassium, copper and the like may be present.
Preferred titanium zeolites will generally have a composition corresponding to the following empirical formula xTiO2(1xe2x88x92x)SiO2 where x is between 0.0001 and 0.5000. More preferably, the value of x is from 0.01 to 0.125. The molar ratio of Si:Ti in the lattice framework of the zeolite is advantageously from 9.5:1 to 99:1 (most preferably from 9.5:1 to 60:1). The use of relatively titanium-rich zeolites may also be desirable.
The catalyst employed in the process of the invention also contains a noble metal. While any of the noble metals can be utilized (i.e., gold, silver, platinum, palladium, iridium, ruthenium, osmium), either alone or in combination, palladium is particularly desirable. Typically, the amount of noble metal present in the catalyst will be in the range of from 0.01 to 20 weight percent, preferably 0.05 to 10 weight percent.
The manner in which the noble metal is incorporated into the catalyst is not considered to be particularly critical. For example, the noble metal may be supported on the titanium zeolite by impregnation, adsorption, ion-exchange, precipitation. Alternatively, the noble metal may be first supported on another support such as an inorganic oxide, inorganic chloride, carbon, or organic polymer resins, or the like, and then physically mixed with the titanium zeolite. Preferred inorganic oxides include oxides of Group 2, 3, 4, 5, 6, 13, or 14 elements. Particularly preferred inorganic oxide supports include silica, alumina, titania, zirconia, niobium oxides, tantalum oxides, molybdenum oxides, tungsten oxides, amorphous titania-silica, amorphous zirconia-silica, amorphous niobia-silica, and the like. Preferred organic polymer resins include polystyrene, styrene-divinylbenzene copolymers, crosslinked polyethyleneimines, and polybenzimidizole. Suitable supports also include organic polymer resins grafted onto inorganic oxide supports, such as polyethylenimine-silica. Preferred supports also include carbon. Particularly preferred supports include carbon, silica, silica-aluminas, titania, zirconia, and niobia.
There are no particular restrictions regarding the choice of noble metal compound used as the source of the noble metal. For example, suitable compounds include the nitrates, sulfates, halides (e.g., chlorides, bromides), carboxylates (e.g. acetate), and amine complexes of noble metals.
Similarly, the oxidation state of the noble metal is not considered critical. In the case of palladium for instance, the palladium may be in an oxidation state anywhere from 0 to +4 or any combination of such oxidation states. To achieve the desired oxidation state or combination of oxidation states, the noble metal compound may be fully or partially pre-reduced after addition to the catalyst. Satisfactory catalytic performance can, however, be attained without any pre-reduction.
After noble metal incorporation, the catalyst is recovered. Suitable catalyst recovery methods include filtration and washing, rotary evaporation and the like. The catalyst is typically dried at a temperature greater than about 50xc2x0 C. prior to use in epoxidation. The drying temperature is preferably from about 50xc2x0 C. to about 300xc2x0 C. The catalyst may additionally comprise a binder or the like and may be molded, spray dried, shaped or extruded into any desired form prior to use in epoxidation. The catalyst may be optionally thermally treated in a gas such as nitrogen, helium, vacuum, hydrogen, oxygen, air, or the like. The thermal treatment temperature is typically from about 50 to about 550xc2x0 C.
The epoxidation process of the invention comprises reacting olefin, oxygen, and hydrogen in the presence of a modifier and the catalyst. The modifier is either (a) calcium carbonate in the presence of carbon dioxide or (b) ammonium bicarbonate. The ammonium bicarbonate or calcium carbonate can be simply added to the reaction medium in which the epoxidation is being performed. The ammonium bicarbonate or calcium carbonate may be added all at once either prior to or following initiation of epoxidation or may be added in an incremental or continuous manner. Ammonium carbonate may also be formed in-situ during the epoxidation reaction.
Sufficient ammonium bicarbonate is necessary to be effective to improve catalyst activity and selectivity to the epoxide as compared to the same reaction carried out under similar conditions in the absence of the ammonium bicarbonate. The amount of ammonium bicarbonate is in the range of from about 10 ppm to about 50,000 ppm (as measured by the weight of ammonium bicarbonate to the weight of the entire reaction mixture), and preferably in the range of from about 100 ppm to about 1500 ppm.
The amount of calcium carbonate used is not believed to be particularly critical, but at a minimum should be effective to improve catalyst activity and selectivity to the epoxide as compared to the same reaction carried out under similar conditions in the absence of the calcium carbonate. Preferably, the amount of calcium carbonate is in the range of from about 50 ppm to about 10,000 ppm (as measured by the weight of calcium carbonate to the weight of the entire reaction mixture), and most preferably in the range of from about 150 ppm to about 5000 ppm.
When the modifier contains calcium carbonate, it must be used in the presence of carbon dioxide. The carbon dioxide will typically be added to the reaction mixture along with the other reaction gases, comprising oxygen, hydrogen, and light olefins such as propylene which may be introduced in the gas phase. Although the amount of carbon dioxide is not critical, the amount of carbon dioxide in the gas is typically in the range of from about 0.01 volume % to about 90 volume % (as measure by volume CO2 per total volume of all gases introduced into the reactor), and preferably from about 0.05 volume % to about 5 volume %.
The epoxidation process of the invention comprises contacting an olefin, oxygen, and hydrogen in the presence of the modifier and the catalyst. Suitable olefins include any olefin having at least one carbon-carbon double bond, and generally from 2 to 60 carbon atoms. Preferably the olefin is an acyclic alkene of from 2 to 30 carbon atoms; the process of the invention is particularly suitable for epoxidizing C2-C6 olefins. More than one double bond may be present, as in a diene or triene for example. The olefin may be a hydrocarbon (i.e., contain only carbon and hydrogen atoms) or may contain functional groups such as halide, carboxyl, hydroxyl, ether, carbonyl, cyano, or nitro groups, or the like. The process of the invention is especially useful for converting propylene to propylene oxide.
Oxygen and hydrogen are also required for the process of the invention. Although any sources of oxygen and hydrogen are suitable, molecular oxygen and molecular hydrogen are preferred. The molar ratio of hydrogen to oxygen can usually be varied in the range of H2:O2=1:100 to 5:1 and is especially favorable at 1:5 to 2:1. The molar ratio of oxygen to olefin is usually 1:1 to 1:20, and preferably 1:1.5 to 1:10. Relatively high oxygen to olefin molar ratios (e.g., 1:1 to 1:3) may be advantageous for certain olefins.
In addition to olefin, oxygen and hydrogen, an inert gas carrier may be preferably used in the process. As the carrier gas, any desired inert gas can be used. Suitable inert gas carriers include noble gases such as helium, neon, and argon in addition to nitrogen and carbon dioxide. Saturated hydrocarbons with 1-8, especially 1-6, and preferably with 1-4 carbon atoms, e.g., methane, ethane, propane, and n-butane, are also suitable. Nitrogen and saturated C1-C4 hydrocarbons are the preferred inert carrier gases. Mixtures of the listed inert carrier gases can also be used. The molar ratio of olefin to carrier gas is usually in the range of 100:1 to 1:10 and especially 20:1 to 1:10.
Specifically in the epoxidation of propylene according to the invention, propane can be supplied in such a way that, in the presence of an appropriate excess of carrier gas, the explosive limits of mixtures of propylene, propane, hydrogen, and oxygen are safely avoided and thus no explosive mixture can form in the reactor or in the feed and discharge lines.
The amount of catalyst used may be determined on the basis of the molar ratio of the titanium contained in the titanium zeolite to the olefin that is supplied per unit time. Typically, sufficient catalyst is present to provide a titanium/olefin per hour molar feed ratio of from 0.0001 to 0.1.
Depending on the olefin to be reacted, the epoxidation according to the invention can be carried out in the liquid phase, the gas phase, or in the supercritical phase. The process of the invention is preferably carried out in the liquid-phase. When a liquid reaction medium is used, the catalyst is preferably in the form of a suspension or fixed-bed. The process may be performed using a continuous flow, semi-batch or batch mode of operation.
If epoxidation is carried out in the liquid phase, it is advantageous to work at a pressure of 1-100 bars and in the presence of one or more solvents. Suitable solvents include, but are not limited to, lower aliphatic alcohols such as methanol, ethanol, isopropanol, and tert-butanol, or mixtures thereof, and water. Fluorinated alcohols can be used. It is also possible to use mixtures of the cited alcohols with water.